The 15 British sailors and marines seized by the Iranian navy yesterday appear to have been well inside Iraqi waters, but they had the ill fortune to stray into the path of the region's aspiring superpower as it flexes new-found muscles.

Wherever Britain and its American allies turn these days in the Middle East, they are bumping into the new realities of Iran's spreading influence.

Today, the United Nations security council is due to vote in New York on new sanctions against Tehran over its nuclear programme, but there is no sign the Iranians will bow to international pressure and surrender the right to enrich uranium.

The Iranian president, Mahmoud Ahmadinejad, who had been due to address the council before the vote, last night cancelled his trip. A spokesman for the Iranian foreign ministry, Mohammed Ali Hosseini, told Iranian state television it was because of "America's obstruction in issuing visas". In Washington, the State Department insisted it had issued 75 visas for Mr Ahmadinejad's delegation, including air crew and support staff.

Iran increasingly appears to be the quartermaster and banker behind the Shia militias keeping British troops pinned down in southern Iraq. In Lebanon, Iran's client Hizbullah holds the key to war or peace, and Tehran is also now a player in the Israeli-Palestinian conflict as a sponsor of Hamas.

The uncomfortable paradox facing London and Washington as they try to put the Iranian genie back in its bottle is that they have done more than anyone to uncork that bottle in the first place and set Iran on the way to regional hegemony.

First they removed the Taliban, Iran's enemy to the east, and then they eliminated Saddam Hussein. In little more than a year, the allies ensured Iran achieved its key strategic objective: to become the dominant power in the Gulf and the Middle East.

"Getting rid of the Taliban and then getting rid of Saddam, basically gave Iran a free ride in the region," said Mamoun Fandy, senior fellow for Gulf security at the International Institute for Strategic Studies. "With the collapse of the Iraqi state, the whole balance of power in the Gulf went out of control, and we moved away from a world of nation states to the world of sectarianism, with Saudi Arabia viewing itself as the centre of gravity of Sunni Islam and by default, Iran became the centre of gravity for Shia Islam."

That worsening sectarian divide has given Iran more influence in Arab states with Shia majorities or significant minorities with a history of subjugation to Sunni rulers: in Iraq, Lebanon, Bahrain and Saudi Arabia. However, that influence in the Arab world transcends the politics of religious identity. For radical Arab Muslims, Tehran is a bastion of defiance to US and British ambitions, and the Iranian nuclear programme is a source of pride.

The nuclear programme also owes much to the invasion of Iraq. The intelligence fiasco over Iraq's non-existent weapons of mass destruction has hamstrung US and British-led efforts to galvanise international action against Iran.

The sanctions due to be voted on today represent only a very tentative extension of measures passed in December, which Iran ignored. They pose no serious threat to Tehran, but represent the lowest common denominator - the most serious package the Russians would accept.

There is talk of military options, but Iran has so far shrugged at such threats. It has seen western military power run into the sand in Iraq.

Britain has an almost demonic image in Iran, dating back to the MI6-backed coup against the nationalist prime minister Mohammad Mossadegh in 1953 and years of British support for the shah, especially in the volatile period before and after the Islamic Revolution of 1979.

In the Khomeini era the traditional hostility was encapsulated in the renaming of the road outside the British embassy in Tehran as "Bobby Sands Avenue" after the IRA hunger striker, who to Iranians symbolised resistance to colonial rule.

Iranian rhetoric still portrays Britain as the "little Satan" alongside the "Great Satan" that is the United States. It is regularly bracketed, with the US and Israel, as hostile to Iranian interests, and stands accused of fomenting recent bomb attacks in border areas.

The two countries' uneasy relationship worsened during the eight-year war with Iraq, when Britain was lambasted for its refusal to supply the arms, ammunition, and spare parts needed by Tehran in its struggle with the "godless Ba'athists of Baghdad". Then came the long crisis over Salman Rushdie, whose novel The Satanic Verses triggered an Iranian "fatwa" permitting him to be killed as an apostate. The author had to live under special branch guard until 1998 when liberalisation in Tehran and the efforts of the late foreign secretary, Robin Cook, brought a negotiated end to the affair.

With the external shackles on Iran's rise removed, its geopolitical ambitions may still be limited by its internal problems. Even the relatively weak UN sanctions appear to have deepened fault lines between the competing centres of power in Tehran, with increasingly tension between President Ahmadinejad and Ayatollah Ali Khamenei. The seizure of British sailors may be an attempt by one faction to force the hand of another.

In Iraq generally, the Iranian leadership must also be careful not to follow the US-British example - and overreach.

Countdown to confrontation

2003 March US invades Iraq and overthrows Saddam Hussein, shuns offer of broad dialogue with Iran.

September UN nuclear watchdog, IAEA, tells Tehran to prove it is not pursuing atomic weapons programme.